DESCRIPTION (from abstract) The overall goals of the proposal are to elucidate some of the fundamental pathways and basic mechanisms for the turnover and degradation of the transmembrane domains (TMD) of type I membrane spanning proteins in the endoplasmic reticulum. Because the gamma- secretase processing of the TMD of the amyloid precursor protein (APP) is critical for the production of the amyloid Abeta peptide and determining the length of the carboxyl terminus of Abeta, the applicants are especially interested in examining pathways that may contribute to the production of the more pathologically relevant Abeta42 in Alzheimer's disease (AD). In order to accomplish these goals, they propose three specific aims that explore the basic mechanisms for TMD proteolysis and degradation, examine their relationship to gamma secretase processing and define the pathways that contribute to Abeta production. In the first specific aim, they will exploit a novel probe that they have designed to specifically examine cleavage events within the membrane. The applicants propose five sub-aims designed to increase our understanding of the basic mechanisms and pathways for the degradation and turnover of the TMD of membrane proteins and to investigate how these pathways may contribute to gamma secretase processing. There are two potential ways in which TMD degradation and gamma secretase processing may be linked: TMD degradation may prevent amyloid production by destroying mis-folded APP substrates that would otherwise give rise to Abeta and perhaps preferentially the more pathological Abeta1-42 form of amyloid. Alternatively, the degradative pathways may give rise to Abeta as a result of partial or incomplete degradation. In the second specific aim, they will analyze the potential role of site 2 protease (S2P) in TMD turnover and APP processing. This protease has recently been identified as the enzyme that cleaves the sterol response element binding protein within the transmembrane domain and preliminary data show that cells deficient in this activity are also deficient in the turnover of the TMD probe. The third specific aim is to characterize the pathways of APP processing and Abeta production in a unique cell line that secretes predominantly (80% of the total Abeta) the more pathological Abeta1-42 form of amyloid. The identification and characterization of the pathways that give rise preferentially to Abeta1-42 may provide insight into the pathological pathways of amyloid production in Alzheimer's disease.